I got asked the question today, what courses have I taken to become a writer? Although my answer was that I had taken three courses at University (the only ones offered at this particular University) I feel that this isn't something a writer should be worried about. In fact, I wrote three novels before ever taking a creative writing class beyond the one available in my high school. I've learned lots since, but it is possible to learn the same things on your own. What you need more than instruction in writing is the desire to write.
Classes are helpful for keeping you on task by giving you deadlines for your work. However, once the classes end, you have to be self motivated enough to set and meet your own goals. This isn't really something that you can be taught in a school.
Just like any writing group, classes can be really helpful for giving you feedback on your work. A good instructor can give you guidance with your stories, point out your weakness in your writing and encourage you to keep trying no matter how hard things get. My first writing instructor has since become a good friend of mine and she still helps me with my writing today. However, it's quite possible that this won't always be the case. When I was considering taking courses, I was worried because I knew that there are a lot of writers in the world who look down on genre fiction.
Your instructors can be very influential over your writing. Since you're goal in school is to get good grades, which is done through appeasing your instructor, there can be a tendency for you to try to write in a way that you know that they like. This causes the risk of creating a lot of people who write in similar styles. You're unique voice is what will separates you're work from others, so obviously if you sound like everyone else, that's a bad thing.
While I think that education is never a bad thing, getting a degree specifically focused on creative writing probably won't help you published and generally won't give you any advantages over other writers. However, it can be a great way to meet professionals and other amateurs that are willing to not only talk to you about writing, but also help you out long after you've finished your schooling. That in itself can be invaluable for someone doing a solitary career such as writing.